Untangling the Mistral and Seasonal Atmospheric Forcing Driving Deep
Convection in the Gulf of Lion: 1993-2013
Abstract
Deep convection occurs periodically in the Gulf of Lion, driven by the
seasonal atmospheric change and Mistral winds. To determine the
variability and drivers of the seasonal and Mistral forcing, 20 years of
ocean simulations were run. Two sets of simulations were performed: a
control set, forced by unfiltered atmospheric forcing, and a seasonal
set, forced by filtered forcing. The filtered forcing retained the
seasonal aspects but removed the high frequency phenomena. Assuming the
Mistral acts primarily in the high frequency, comparing the two sets
allows for distinguishing the effects of the Mistral on the ocean
response. During the preconditioning phase, the seasonal forcing was
found to be the main destratifying process, removing on average 45.7%
of the stratification, versus the 28.0% removed by the Mistral. Despite
this difference, at the time of deep convection, both the seasonal and
Mistral forcing each triggered deep convection in roughly half of the
events. Larger sensible and latent heat fluxes were found in the
seasonal forcing of the years with deep convection, acting as the main
drivers (removing 0.17 m2s-2 and 0.43 m2s-2 of stratification,
respectively). They are themselves driven by increased wind speeds,
believed to be the low frequency signal of the Mistral, as more Mistral
events occur during winters with deep convection (34.3% versus 28.6%).
The evolution of the seasonal forcing in a changing climate may have a
significant effect on the future deep convection cycle of the Gulf of
Lion.